Here is an illustration that I have made of a digging box for ratties. As a substrate, I use organic indoor potting soil without any fertilisers. Remember that there is NO soil in potting soil but only organic matter like bark and leaves. Even my rat with all his allergies is perfectly fine with it. You are welcome to copy this. Just keep the soil moist to prevent any dust particles. Replace once a month. Let me know how your rats react to it! Be extremely careful of flying soil ...LOL.

Warning: You will be sorry if you do not install the soil trap door ...LOL